Electrodeposition can be carried out with a device or system which comprises an anode and a cathode in contact with an electrolyte, and an electric current source connected across the anode and cathode to induce an ion migration in the electrolyte of ions of one charge to an electrode of the opposite charge. When the ions in the electrolyte come into contact with the electrode of the opposite charge, a charge neutralization occurs and a material from which the ions were constituted, can be transferred out of the electrolyte by a "deposition" whether this deposition results in electrodeposition or plating of the material onto the electrode in question, results in a release of the material as a gas, or otherwise effects a change in the state of the material. Hence the term "deposition" is used in its broadest possible sense to refer to the act of withdrawal from the electrolyte of a material which may have been present in a different form, i.e. as ions, in this electrode.
An electrolytic deposition itself generally comprises a vessel holding an electrolyte bath and an anode and cathode immersed in or in contact with the electrolyte. These electrodes are connected in a current-conducting path, usually making use of an external voltage source connected to the electrodes, to effect a separation of material from the electrolyte on one or both of the electrodes. As noted previously, the separated material can be a gas, a liquid or a solid.
While the electrolyte is usually a liquid, it can have any viscosity ranging from an extremely low viscosity to an extremely high viscosity as long as it sustains the iontransport phenomenon which results in the separation of material from the electrolyte in the manner previously described. For example, the electrolyte can be a gel which is practically nonflowable but is capable of sustaining the transport phenomenon.
The electrolytic separation cells are provided for a wide variety of approaches, purposes or processes. For example, they may be used for galvanization, i.e. electrocoating of a material upon a conductive substrata, for electrodeposition in the purification of metals, for electrolysis (gas generation) and for the production, recovery or purification of various chemical elements or substances.
The invention is concerned especially with electrochemical processes which take place at electrodes in contact with an electrolyte in electrolytic deposition cells. It is known that the combination of each electrode/electrolyte has a characteristic electrode potential as thermodynamically determined by the so-called Nernst equation. Further (see DETTNER-ELZE "HANDBUCH DER GALVANOTECHNIK, MuNCHEN", 1963, Vol. 1 Part 1, pp. 35 ff.).
Deviations from the electrode potential are dependent upon the current amplitude and are a result of kinetic barriers to a greater or lesser extent. These differences between the actual electrode potential and the theoretical electrode potential as determined by the Nernst equation are referred to as overpotential or overvoltage. The overvoltage depends upon various factors. One of these factors is the barrier to penetration by the current carrier through the HELMHOLTZ double layer at the boundary between the electrode and the electrolyte. The portion of the overvoltage which is a result of this penetration barrier can be referred to as penetration overvoltage. It depends upon the current density at the electrode and, in an as yet incompletely defined manner, upon the nature of the ions which are released from the electrolyte and upon the construction and material of the electrode in contact therewith.
It is known (see CHEMIE-INGENIEUR TECHNIK, 1966, Page 643 to 648) to provide electrodes for electrolysis which are composed of semiconductive materials. Investigations of this type are directed to determinations of the influence of the electronic structure of a solid body upon reactions taking place on its surface so as to clarify the nature of electrode surfaces and the phase boundaries between electrodes and electrolytes. Such investigations have, for example, studied the system selenium/sulfuric acid to determine the effect of the electrode surface upon the penetration overvoltage (see ZEITSCHRIFT FUR ELEKTROCHEMIE-BERICHTE DER BUNSENGESELLSCHAFT FUR PHYSIKALISCHE CHEMIE, 1959, Pgs. 541-550). These investigations have indicated that the penetration overvoltage is a consequence of the reduced supply of current carriers with the electrode at the phase boundary.
It is also known that, in electrolytic deposition, the electrical energy used per mole of the deposited material is proportional to the applied voltage, the current flowing the system and the time. The penetration overvoltage increases the voltage necessary to pass a given current through this system and therefore causes an increase in the specific energy consumption, i.e. the energy comsumption per mole of deposited material.